1. Field of the Invention
The present invention relates to a discharge lamp that is used as a light source in, for example, a projector, and more particularly to an ac-lighting type ultra-high pressure mercury lamp and to a lighting method.
2. Description of the Related Art
In an ac-lighting type of ultra-high pressure mercury lamp, lighting properties have a great influence on the characteristics of the lamp (life and luminance). For example, once lighted, the interior of the lamp reaches a high pressure, and the lamp is therefore extremely difficult to relight immediately after being extinguished. This deterioration of the lighting property both increases the consumption of the lamp electrode, this consumption being caused by sputtering that occurs when an arc-spot is formed, and tends to reduce luminance.
A lighting method is proposed in Japanese Patent Laid-Open Publication No. 2002-289379 for improving the lighting property when relighting a lamp. In this lighting method, during the course from the lit state to the extinguished state of an ultra-high pressure discharge lamp in which mercury is sealed inside an arc tube, the lamp power that is supplied to electrodes inside the arc tube is first reduced to a level such that the arc discharge is not extinguished, and after this reduced state is maintained for a set time period, the supply of current is cut off. This method both reduces the build-up of mercury on the surfaces of the two lamp electrodes and prevents consumption of the lamp electrodes caused by sputtering that occurs in the time interval from start-up until arc generation becomes stabilized.
One cause for the deterioration of the lighting property in an ac-lighting type of ultra-high pressure mercury lamp is the build-up of mercury on the electrodes when the lamp is extinguished. The state of build-up of mercury on the electrodes varies with such factors as the electrode configuration of the lamp, the distance between the electrodes and the inner surface of the bulb, and the rate of cooling. For example, metal components for incorporation in a reflector are provided on the side of one electrode in an ultra-high pressure mercury lamp, and the cooling of this electrode after the lamp is extinguished is therefore faster than the cooling of the other electrode. In this case, the build-up of mercury is advanced on the electrode that experiences faster cooling. In extreme cases, it is conceivable that all of the mercury will build up on one electrode. When most of the mercury builds up on one electrode in this way, the lamp tends not to light up smoothly.
In the lamp that is disclosed in the above-described patent document, the electrodes can be maintained at a particular temperature by maintaining the formation of the arc over a fixed time interval after the lamp is turned off, whereby nearly all of the mercury can be caused to build up on the inner surface of the bulb, which cools quickly. However, in this method there is the concern that chemical compounds will also be deposited when the mercury builds up on the inner surface of the bulb and thus diminish luminance.